The present invention relates to a catalytic burner arrangement, as well as to an auxiliary power assembly.
In auxiliary power units based on a fuel cell technology, energy is provided by a fuel cell stack. For the operation of the fuel cell usually hydrogen is used. In said APU systems hydrogen is usually produced by so called fuel reformers which generate a hydrogen rich gas from hydrocarbon fuels, like diesel, by means of a catalyst. In some preferred fuel reforming processes, as the autothermal fuel reforming process or the steam reforming process, steam is additionally used for the fuel reforming reaction. The heat required for the production of steam may be provided by use of a catalytic burner arranged downstream of the fuel cell or fuel cell stack, wherein air and excess hydrogen exiting the fuel cell stack are combusted over a catalyst to release energy, which can be used for the steam production.
The known catalytic burners have a housing defining a reaction chamber with an inlet for fuel (hydrogen) and an inlet for oxidant (air), whereby fuel and oxidant are introduced into the reaction chamber. The housing further incorporates a catalyst, which is arranged downstream of the inlets, where hydrogen and air catalytically react with each other. The problem of the known catalytic burners is that air and hydrogen often react uncontrolled upstream of the catalyst as soon as being brought in contact with each other. In some cases air may even enter the fuel inlet, whereby such an uncontrolled combustion may also take place in the pipes. However, these uncontrolled combustions may damage the pipes as well as the burner itself. Additionally, often the mixing of air and fuel is inhomogeneous, which in turn results in the development of hotspots in the catalyst, which might damage the catalyst and produce unwanted emissions.
It is desirable to provide a catalytic burner, which hinders ignition of the hydrogen in the pipes and provides a homogenous mixture of air and fuel.
According to aspects of the present invention, a catalytic burner according to as well as an auxiliary power unit assembly are provided.
In the following a catalytic burner arrangement is provided which comprises at least a catalytic burner unit and a mixing unit. Thereby, the catalytic burner unit comprises a housing which defines a reaction chamber in which a catalyst is arranged. The catalyst is adapted to react a fuel, particularly a hydrogen containing fluid with an oxidant, particularly air, for producing heat. The housing further has a fluid inlet for supplying a fluid stream into the housing and a fluid outlet for exiting a fluid stream from the housing.
The mixing unit in turn forms a mixing chamber in which fuel and oxidant are mixed and comprises a fuel inlet and an oxidant inlet as well as a fuel-oxidant-mixture outlet. The fuel inlet of the catalytic burner unit merges with the fuel-oxidant-outlet of the mixing unit so that the fuel-oxidant-mixture from the mixing chamber may be transported to the reaction chamber of the catalytic burner unit.
In order to hinder the fuel and the oxidant reacting uncontrolled with each other and providing an improved mixing, said fuel-oxidant-outlet of the mixing chamber is pipe-shaped and extents into the mixing chamber of the mixing unit. By means of the pipe-shaped fuel-oxidant-outlet extending into the mixing chamber, fuel and oxidant are guided in a swirl around the fuel-oxidant-outlet and are forced to stream upwards and to change stream direction before the fuel/oxidant mixture may enter the fuel-oxidant-outlet.
It should be noted that “pipe-shaped” in the context of the present invention refers to an elongated hollow element, which may have a cylindrical or prismatic form. Said hollow element has at least two openings. At least one first opening allows an entrance of the fuel-oxidant mixture into the hollow element and at least one second opening allows an exit of the fuel-oxidant mixture from the hollow element and thereby from the mixing unit. Thereby, the at least one first opening is arranged inside the mixing chamber. It should be further explicitly noted that more than one opening as first opening and more than one opening as second opening may be provided.
According to an alternate solution, said fuel inlet of the mixing chamber is arranged upstream of said oxidant inlet. This staggered arrangement of the inlets prevents the oxidant from entering the fuel inlet and thereby prevents an uncontrolled ignition of the fuel. Even if it is preferred to provide in addition a pipe-shaped fuel-oxidant-outlet of the mixing chamber which extends into the mixing chamber of the mixing unit, the staggered arrangement alone also provides an improved mixing and prevents uncontrolled combustion.
According to a preferred embodiment, a length of the pipe-shaped fuel-oxidant-outlet extents over the oxidant inlet and/or the fuel inlet. Thereby, it may be preferred if the fuel-oxidant-outlet extends over both the oxidant inlet and the fuel inlet. In both embodiments, the swirl and the stream redirection may be maximized.
According to a further preferred embodiment, the fuel inlet of the mixing chamber is arranged upstream of said oxidant inlet. Thereby, the oxidant is reliably hindered from entering the fuel inlet and reacting uncontrolled.
According to a further preferred embodiment, the fuel inlet and oxidant inlet are arranged angled to a direction of a main fluid stream streaming through the fuel-oxidant-mixture outlet to the reaction chamber of the catalytic burner. Advantageously, the angled arrangement provides a homogenous mixture as the fluid needs to be redirected from the entrance direction to its exit direction, whereby a mixing of the fluids is performed.
For having a directed fluid stream of a fuel and oxidant, it is preferred if the fuel inlet and/or the oxidant inlet are designed as at least one pipe having a longitudinal axis, whereby the directed fluid streams are provided.
According to a further preferred embodiment, said mixing unit is prismaticly or cylindrically shaped having two basis plates and at least three side surfaces or a mantel side, wherein the fuel inlet and the oxidant inlet are arranged in the side surfaces or in the mantel side, and the fuel-oxidant-mixture outlet is arranged at one of the basis plates. Thereby, the geometric design of the mixing unit supports the mixing so that a very homogenous mixture may be provided.
According to a further preferred embodiment, at least one of the directed fluid streams are offset from a longitudinal axis of the mixing chamber, whereby at least one tangential fluid stream is provided. By means of the tangential fluid streams a homogenous mixture may be achieved.
According to a further preferred embodiment, the longitudinal axis of the fuel inlet and/or of the oxidant inlet are inclined to a cross sectional plane of the mixing chamber. By the inclined arrangement an uncontrolled ignition of oxidant and fuel and/or an unwanted entering of oxidant into the fuel pipe is avoided.
According to a further preferred embodiment, the oxidant inlet and the fuel inlet are arranged substantially rectangular to each other, whereby both the mixing is improved and an unwanted ignition is reliably avoided.
A further aspect of the present application relates to an auxiliary power assembly based on fuel cell technology which comprises at least a fuel processing assembly which is adapted to convert hydrocarbon fuels into a hydrogen rich gas for fuel cells by using at least hydrocarbon fuel and steam. Downstream of the processor assembly at least one fuel cell or fuel cell stack for providing auxiliary power is arranged. Downstream of the fuel cell a catalytic burner unit is provided which is adapted to burn unused hydrogen exiting from the fuel cell or the fuel cell stack by using an oxidant, such as air or oxygen, and a catalyst for reacting said oxidant and hydrogen to heat, wherein said heat in turn is used for the production of steam used in the fuel processing assembly. Thereby the catalytic burner is designed as described above.
Further embodiments and preferred arrangements are defined in the description, the figures and the attached claims.
In the following same or similarly functioning elements are indicated with the same reference signs.